“Server Error in '/' Application.”
The application behind the site threw an exception it didn't handle, and IIS replaced the page with its yellow error screen. In production the screen hides the details ("Runtime Error" with a note about customErrors); on a development setting it shows the stack trace, which also leaks server paths to visitors. Either way nobody sees your content.
Is your site showing it right now?
The free scan loads your page like a visitor and checks for this message, plus broken links, script errors and phone layout. It also walks your signup, booking or contact form, without sending anything.
Free, no signup. Usually done in about a minute.Why it happens
- A code or configuration deploy with a bug, or a web.config that doesn't parse (a stray character is enough)
- A missing or wrong-version DLL after a deploy, or a NuGet package the server doesn't have
- The database connection string is wrong or the database is unreachable
- The application pool is on the wrong .NET version, or it stopped after repeated crashes
- File permissions on the site folder changed, so the app can't read or write where it needs to
How to fix it
Read the real error
On the server, open Event Viewer (Windows Logs, Application) or the app's own log. The first exception after the deploy names the file or setting. If you can't reach the server, ask your host for the application log.
Check web.config
If the error is a configuration error, open web.config and look at the line it names. Undo the last change, or restore the previous copy from your deploy.
Restart the application pool
In IIS Manager, select the site's application pool and choose Recycle (or Start if it stopped). A pool that stops repeatedly means the app crashes on startup, so fix the cause too.
Redeploy the previous build
If it started with a deploy, publish the last working build again, including its bin folder, and check that every dependency is included.
Hide details from visitors
In web.config, set <customErrors mode="RemoteOnly"> (or mode="On") so visitors never see stack traces, and give them a friendly error page with defaultRedirect.
Questions
Visitors see a yellow page with code on it. Is that bad?
Yes, beyond looking broken: the stack trace shows file paths, framework versions and sometimes query details. Set customErrors to RemoteOnly so only someone on the server sees it.
Only one page shows the error
Then the crash is in that page's code or a query it runs. The event log entry for that page names the exception.
Can Page Canary tell me if this comes back?
Yes. Page Canary checks your key pages every day for this message and others like it, confirms it on a second check, and emails you with the fix. One site is free.